Liquid crystal display device

ABSTRACT

A liquid crystal display device of the present invention includes: a liquid crystal layer containing a nematic liquid crystal material; a pair of electrodes opposing each other via the liquid crystal layer; a pair of alignment films respectively provided between the pair of electrodes and the liquid crystal layer; and an alignment sustaining layer formed of a photopolymerized material on each of surfaces of the pair of alignment films which are closer to the liquid crystal layer, the alignment sustaining layer being configured to regulate a pretilt azimuth of a liquid crystal molecule of the liquid crystal layer during the absence of an applied voltage across the liquid crystal layer, wherein the pretilt azimuth of the liquid crystal molecule of the liquid crystal layer is regulated by the alignment sustaining layer during the absence of an applied voltage across the liquid crystal layer. The nematic liquid crystal material contains a liquid crystal compound having a terphenyl ring system as an indispensable component, and the liquid crystal layer further contains part of a photopolymerizable compound which is a source material of the photopolymerized material, a content of the photopolymerizable compound relative to the nematic liquid crystal material being less than 0.015 mol %.

TECHNICAL FIELD

The present invention relates to a liquid crystal display device andspecifically to a liquid crystal display device which uses the PolymerSustained Alignment Technology.

BACKGROUND ART

Liquid crystal display devices perform display by utilizing the changeof the orientations of liquid crystal molecules which is caused by avoltage applied across the liquid crystal layer. The orientations of theliquid crystal molecules which occur in the absence of an appliedvoltage across the liquid crystal layer (pretilt directions) areconventionally regulated by alignment films. For example, in TN-modeliquid crystal display devices, the pretilt azimuths of the liquidcrystal molecules are regulated by performing a rubbing process onhorizontal alignment films. In this specification, the pretilt azimuthrefers to a component of a vector that is indicative of the orientationof a liquid crystal molecule in the liquid crystal layer in the absencean applied voltage, the component being in a plane of the liquid crystallayer (in a plane of the substrate). The pretilt angle, which is anangle formed by the alignment film and the liquid crystal molecule, isdetermined depending primarily on a combination of the alignment filmand the liquid crystal material. The pretilt direction is expressed bythe pretilt azimuth and the pretilt angle. In TN-mode liquid crystaldisplay devices, the pretilt azimuths regulated by a pair of alignmentfilms which oppose each other via the liquid crystal layer are setperpendicular to each other. The pretilt angle is about 1° to 5°.

In recent years, as a technology for controlling the pretilt directionsof the liquid crystal molecules, the Polymer Sustained AlignmentTechnology (hereinafter, referred to as “PSA technology”) has beendeveloped (see Patent Documents 1, 2, and 3). The PSA technology is atechnology of controlling the pretilt directions of the liquid crystalmolecules by means of a polymer formed in the liquid crystal layer. Thepolymer is formed by irradiating, after assemblage of a liquid crystalcell, a small amount of polymerizable material (e.g., aphotopolymerizable monomer) mixed in a liquid crystal material withactive energy rays (e.g., ultraviolet light) while a predeterminedvoltage is applied across the liquid crystal layer. The orientations ofthe liquid crystal molecules maintained during the formation of thepolymer are sustained (memorized) even after removal of the voltage (inthe absence of an applied voltage). Thus, the PSA technology isadvantageously capable of adjusting the pretilt azimuths and pretiltangles of the liquid crystal molecules by controlling, for example, anelectric field generated in the liquid crystal layer. Also, the PSAtechnology does not require a rubbing process and is therefore suitableto formation of a vertical alignment type liquid crystal layer that hasdifficulty in regulating the pretilt directions by means of a rubbingprocess. The entire disclosures of Patent Documents 1, 2, and 3 areincorporated in this specification by reference.

Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-357830

Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-307720

Patent Document 3: Japanese Laid-Open Patent Publication No. 2006-78968

Patent Document 4: Japanese Laid-Open Patent Publication No. 2006-169518

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, in the PSA technology, an insufficient UV dose leads to aproblem that an unreacted portion of a photopolymerizable compoundremains in the liquid crystal layer. The remaining photopolymerizablecompound gradually reacts due to light or heat from a backlight duringuse of the liquid crystal display device, so that the orientations ofthe liquid crystal molecules which occur during the use may be sustained(memorized). If this phenomenon occurs, it may emerge in the form ofimage sticking. An image sticking phenomenon which is detrimental togeneral liquid crystal display devices is caused by a DC componentremaining in the liquid crystal layer and is therefore called “DC imagesticking”, while the above-described image sticking phenomenon whichparticularly occurs in connection with the PSA technology is hereinreferred to as “polymeric image sticking”.

The ultraviolet light can damage the liquid crystal material and theorganic materials of the alignment films and other constituents and cantherefore decrease the reliability. Thus, the UV dose cannot beinordinately increased. As the duration of the ultraviolet irradiationincreases, the tact time accordingly increases. This leads to anincrease in production cost.

Patent Document 3 discloses that the amount of a residual monomer ispreferably 0.02 mass % or less. However, in example 6 (table 2) whichuses a liquid crystal display panel, the amount of the residual monomercannot be decreased to 0.017 mass % without ultraviolet irradiation ofmore than 40 J/cm². Even when the UV dose is increased from 30 J/cm² to40 J/cm² in the second ultraviolet irradiation step, the amount of theresidual monomer only decreases from 0.02 mass % to 0.017 mass %. Thus,it is understood that, in consideration of reliability and productioncost, the decrease in the amount of the residual monomer by means ofincreasing the UV dose has a limit.

The present invention was conceived in order to solve the aboveproblems. One of the objects of the present invention is to provide aliquid crystal display device fabricated using the PSA technology, inwhich the amount of a polymerizable compound remaining in the liquidcrystal layer is sufficiently decreased without increasing the UV doseas compared with the conventional process.

Means for Solving the Problems

A liquid crystal display device of the present invention includes: aliquid crystal layer containing a nematic liquid crystal material; apair of electrodes opposing each other via the liquid crystal layer; apair of alignment films respectively provided between the pair ofelectrodes and the liquid crystal layer; and an alignment sustaininglayer formed of a photopolymerized material on each of surfaces of thepair of alignment films which are closer to the liquid crystal layer,the alignment sustaining layer being configured to regulate a pretiltazimuth of a liquid crystal molecule of the liquid crystal layer duringthe absence of an applied voltage across the liquid crystal layer,wherein the pretilt azimuth of the liquid crystal molecule of the liquidcrystal layer is regulated by the alignment sustaining layer during theabsence of an applied voltage across the liquid crystal layer, thenematic liquid crystal material contains a liquid crystal compoundhaving a terphenyl ring system as an indispensable component, and theliquid crystal layer further contains part of a photopolymerizablecompound which is a source material of the photopolymerized material, acontent of the photopolymerizable compound relative to the nematicliquid crystal material being less than 0.015 mol %.

In one embodiment, a content of the liquid crystal compound having theterphenyl ring system in the nematic liquid crystal material is in arange of not less than 1 mol % and not more than 25 mol %.

In one embodiment, the photopolymerizable compound includes a diacrylatemonomer which has a liquid crystal skeleton or a dimethacrylate monomerwhich has a liquid crystal skeleton.

In one embodiment, the pair of alignment sustaining layers include aparticle of the photopolymerized material which has a particle diameterof 50 nm or less.

In one embodiment, the pair of alignment films are vertical alignmentfilms, and the nematic liquid crystal material has negative dielectricanisotropy.

Effects of the Invention

In a liquid crystal display device of the present invention, a nematicliquid crystal material included in a liquid crystal layer contains aliquid crystal compound having a terphenyl ring system as anindispensable component. The liquid crystal compound having a terphenylring system has the function of increasing the efficiency of apolymerization reaction of a photopolymerizable compound in the liquidcrystal layer. Therefore, in an end-product liquid crystal displaydevice, the content of the photopolymerizable compound remaining in theliquid crystal layer relative to the nematic liquid crystal material canbe less than 0.015 mol %. Further, the UV dose need not to be increasedas compared with the conventional process. Thus, the liquid crystaldisplay device of the present invention is also advantageous in terms ofreliability and production cost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 A cross-sectional view of one pixel of a liquid crystal displaydevice 100. (a) schematically shows an alignment of liquid crystalmolecules 42 a in a black display state (in the absence of an appliedvoltage). (b) schematically shows an alignment of liquid crystalmolecules 42 a in a white display state (in the presence of an appliedvoltage).

FIG. 2 A SEM image of an alignment sustaining layer included in a liquidcrystal display device of an embodiment of the present invention.

FIG. 3 A graph for illustrating the relationship between the compositionof a nematic liquid crystal material and the residual monomer ratio. Theordinate axis represents the residual monomer ratio. The abscissa axisrepresents the temperature of a liquid crystal cell during ultravioletirradiation.

FIG. 4 A table showing liquid crystal compounds which are constituentsof nematic liquid crystal materials LC-5, LC-6 and LC-7 used inexperiments, the chemical structure formula, the composition, and thedielectric anisotropy (Δ∈).

FIG. 5 A graph showing the relationship between the content of aterphenyl liquid crystal compound and the residual monomer ratio.

FIG. 6 A table showing liquid crystal compounds which are constituentsof nematic liquid crystal material LC-9 used in an experiment, thechemical structure formula, the composition, and the dielectricanisotropy (Δ∈).

FIG. 7 A graph showing the relationship between the content of aterphenyl liquid crystal compound and the voltage holding ratio (theinitial values and the values reached after ultraviolet irradiation).

DESCRIPTION OF THE REFERENCE NUMERALS

-   -   11, 21 substrate    -   12 pixel electrode    -   22 counter electrode    -   22 a opening    -   34 a, 34 b alignment sustaining layer    -   42 liquid crystal layer    -   42 a liquid crystal molecules    -   100 liquid crystal display device

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a liquid crystal display device of an embodiment of thepresent invention is described with reference to the drawings. Thepresent invention is not limited to the embodiment described below.

[Liquid Crystal Display Device Fabricated Using PSA Technology]

First, a structure and operation of a liquid crystal display devicefabricated using the PSA technology are described with reference to FIG.1 and FIG. 2.

FIG. 1 is a cross-sectional view schematically showing a structure ofone pixel of a liquid crystal display device 100 of an embodiment of thepresent invention. FIG. 1( a) shows an alignment of liquid crystalmolecules 42 a in a black display state (in the absence of an appliedvoltage). FIG. 1( b) shows an alignment of the liquid crystal molecules42 a in a white display state (in the presence of an applied voltage).Hereinafter, the embodiment of the present invention is described withan example of a vertical alignment type liquid crystal display devicewhich performs display in a normally black mode, to which the presentinvention is however not limited.

The liquid crystal display device 100 has a plurality of pixels andincludes a pair of substrates 11 and a pair of polarizing plates (notshown) placed in crossed Nicole on the outer sides of the substrates.The liquid crystal display device 100 is configured to display images ina normally black mode. Each pixel has a liquid crystal layer 42including a nematic liquid crystal material (liquid crystal molecules 42a) whose dielectric anisotropy is negative, and a pixel electrode 12 anda counter electrode 22 which oppose each other via the liquid crystallayer 42. A pair of vertical alignment films (not shown) arerespectively provided between the pixel electrode 12 and the liquidcrystal layer 42 and between the counter electrode 22 and the liquidcrystal layer 42. Surfaces of the alignment films which are closer tothe liquid crystal layer 42 are respectively provided with a pair ofalignment sustaining layers 34 a and 34 b formed of a photopolymerizedmaterial. The alignment sustaining layers 34 a and 34 b are formed by,after formation of a liquid crystal cell, polymerizing aphotopolymerizable compound contained in a prepared liquid crystalmaterial while a voltage is applied across the liquid crystal layer 42.

Before the polymerization of the photopolymerizable compound, thealignment of the liquid crystal molecules 42 a is controlled by thevertical alignment films (not shown) so that the liquid crystalmolecules 42 a are oriented vertically to the substrate surface. When awhite display voltage is applied, the liquid crystal molecules 42 aresult in an alignment where they are inclined in predetermineddirections according to an oblique electric field generated at an edgeportion of the pixel electrode 12 and an oblique electric fieldgenerated near an opening 22 a of the counter electrode 22 as shown inFIG. 1( b).

The alignment sustaining layers 34 a and 34 b, which are formed underapplication of the white display voltage, function to sustain (memorize)an alignment of the liquid crystal molecules 42 a which occurs underapplication of the white display voltage across the liquid crystal layer42 even after removal of the voltage (in the absence of an appliedvoltage) as shown in FIG. 1( a).

The liquid crystal molecules 42 a in the closest vicinity of thevertical alignment films are under the strong anchoring effect and aretherefore oriented vertical to the surfaces of the vertical alignmentfilms even in the presence of an applied voltage. Thus, the liquidcrystal molecules 42 a fixed by the alignment sustaining layers 34 a and34 b formed over the vertical alignment films only have a slight tilt(1° to 5°) from the vertical direction (pretilt angle of 85° to 89°) asschematically shown in FIG. 1( a). As seen from the comparison of FIG.1( a) and FIG. 1( b), the alignment of the liquid crystal molecules 42 afixed by the alignment sustaining layers 34 a and 34 b scarcely changeeven when a voltage is applied.

The liquid crystal display device 100 of an embodiment of the presentinvention has the alignment sustaining layers 34 a and 34 b andtherefore exhibits an alignment of the liquid crystal moleculespretilted in predetermined directions as shown in FIG. 1( a) even in theabsence of an applied voltage. The alignment which occurs in thiscondition conforms to the alignment of the liquid crystal molecules 42 awhich occurs in a white display state (in the presence of an appliedvoltage) as shown in FIG. 1( b). As a result, a stable alignment can beachieved, and the response characteristics of the liquid crystalmolecules, etc., can be improved.

In the example described herein, an opening 22 a (portion not includinga conductive layer) is provided in the counter electrode 22 in order tocontrol the orientations of the liquid crystal molecules 42 a. However,the method for controlling the orientations of the liquid crystalmolecules 42 a in the formation of the alignment sustaining layers 34 aand 34 b is not limited to this example. For example, as described inPatent Document 1, by providing a dielectric protrusion or electrodeslit as necessary, four liquid crystal domains which have differentazimuths of the orientations of the liquid crystal molecules 42 a can beformed.

The alignment sustaining layers 34 a and 34 b can be formed, forexample, in a manner described below. Here, a liquid crystal displaypanel is fabricated according to the same method as that described inPatent Document 3 (example 6).

A liquid crystal display panel for use in the liquid crystal displaydevice 100 is fabricated using a material in which a photopolymerizablecompound of not less than 0.1 mass % and not more than 0.5 mass % ismixed in a nematic liquid crystal material of negative dielectricanisotropy. The photopolymerizable compound may preferably be a monomeror oligomer which has a radically-polymerizable functional group, suchas an acrylate group, a methacrylate group, a vinyl group, or the like.In terms of reactivity, a monomer or oligomer which has an acrylategroup or a methacrylate group is more preferable. Among such examples, apolyfunctional group is preferable. The photopolymerizable compound usedherein may be a diacrylate or dimethacrylate monomer which has a liquidcrystal skeleton. By using a monomer which has a liquid crystalskeleton, the alignment of the liquid crystal molecules can be sustainedmore stably. Especially, a ring system or condensed ring systemdescribed in Patent Document 2 to which an acrylate group or amethacrylate group is directly bonded is preferable.

The liquid crystal layer of this liquid crystal display panel (includingthe above-described monomer) is irradiated with UV light (e.g., i-lineat the wavelength of 365 nm, about 20 mW) at about 20 J/cm² while apredetermined voltage is applied across the liquid crystal layer.Application of the voltage across the liquid crystal layer causes theliquid crystal molecules 42 a to have a predetermined alignmentaccording to electric fields generated between the counter electrode 22and the pixel electrode 12. The UV irradiation causes polymerization ofthe photopolymerizable compound so that a photopolymerized material isproduced. The photopolymerized material forms the alignment sustaininglayers 34 a and 34 b on the vertical alignment films for fixing theabove alignment of the liquid crystal molecules 42 a. A series of stepsfor photopolymerizing a photopolymerizable compound while apredetermined voltage which is not lower than the white display voltageis applied to form alignment sustaining layers is sometimes referred toas “PSA process”.

An example of the alignment sustaining layers 34 a and 34 b is describedas to the structure with reference to FIG. 2. A SEM image shown in FIG.2 is a result of a SEM observation of a surface of the alignmentsustaining layer. Specifically, a sample of a liquid crystal displaypanel fabricated as described above was disassembled, and thereafter,the liquid crystal material was removed from the disassembled sample. Asurface of the alignment sustaining layer of the resultant sample wasthen washed with a solvent and observed by SEM.

As seen from FIG. 2, the alignment sustaining layer contains particlesof the photopolymerized material with the particle size of 50 nm orless. The photopolymerized material may not necessarily cover the entiresurfaces of the alignment films. The surfaces of the alignment films maybe partially exposed. The liquid crystal molecules aligned according toelectric fields generated in the liquid crystal layer are fixed by thephotopolymerized material, so that the alignment of the liquid crystalmolecules is sustained even in the absence of an electric field. Afterthe formation of the alignment sustaining layers over the verticalalignment films, the alignment sustaining layers regulate the pretiltdirections of the liquid crystal molecules.

The presence of the alignment sustaining layers can also be confirmedusing a surface analysis method different from SEM, such as AFM, SIMS,or the like.

[Liquid Crystal Compound Having Terphenyl Ring System]

The present inventor found that, when the nematic liquid crystalmaterial contains a liquid crystal compound having a terphenyl ringsystem (which refers to a para isomer unless otherwise specified), theamount of the residual photopolymerizable compound is reduced, andreached the concept of the present invention. Note that the liquidcrystal compound having a terphenyl ring system is hereinafter referredas “terphenyl liquid crystal compound”. Hereinafter, the presentinvention is described below in details with experiment examples. Notethat Patent Document 4 discloses a nematic liquid crystal material ofnegative dielectric anisotropy which contains a terphenyl liquid crystalcompound as an optional component, but does not in the least disclose orsuggest the knowledge described below.

The relationship between the composition of the nematic liquid crystalmaterial and the residual monomer ratio is described with reference toFIG. 3. Liquid crystal material LC-1 used herein is a nematic liquidcrystal material whose dielectric anisotropy (Δ∈) is zero. LC-2 to LC-4are mixtures of LC-1 (primary material) and different types of liquidcrystal compounds. Specifically, each of LC-2 to LC-4 contains LC-1 andone type of the liquid crystal compounds in the proportion of 15 mass %.LC-2 is a mixture of LC-1 and terphenyl liquid crystal compound A. LC-3is a mixture of LC-1 and terphenyl liquid crystal compound B. LC-4 is amixture of LC-1 and an alkenyl liquid crystal compound.

In liquid crystal materials LC-1 to LC-4, 0.3 mass % of a dimethacrylatemonomer having a liquid crystal skeleton was mixed as thephotopolymerizable compound to prepare liquid crystal materials for PSA.Note that a photoinitiator was not used.

The resultant liquid crystal materials for PSA were used to fabricateliquid crystal cells (cell gap: 3.25 μm) which were then irradiated withultraviolet light (365 nm) of about 23 mW/cm² at about 400 mJ/cm² at 20°C., 30° C., 40° C., and 50° C. Thereafter, each of the liquid crystalcells was disassembled, and a small amount of the liquid crystalmaterial was sampled. The sampled liquid crystal material was dilutedwith acetone, and the residual monomer was quantified by a gaschromatography/mass spectrometry (GC/MS) method. In the graph shown inFIG. 3, the ordinate axis represents the residual monomer ratio, whichis indicative of the ratio of the amount of the residual monomer to theinitial monomer amount in percentages. The abscissa axis represents thetemperature of the liquid crystal cell during ultraviolet irradiation.Note that the present inventor experimentally confirmed that the sameresults are obtained even when a diacrylate monomer was used insubstitution for the dimethacrylate monomer.

As clearly seen from FIG. 3, the residual monomer ratio of LC-4 whichcontains an alkenyl liquid crystal compound is not largely differentfrom that of LC-1 which is the primary material of LC-4. On the otherhand, LC-2 and LC-3, each of which contains a terphenyl liquid crystalcompound, have smaller residual monomer ratios than that of LC-1. Asclearly understood from this, the terphenyl liquid crystal compoundspecifically contributes to the reaction of the photopolymerizablecompound and produces the effect of decreasing the amount of theresidual monomer. This effect is produced at substantially the samedegree irrespective of the type of the terphenyl liquid crystal compound(A or B). As understood from FIG. 3, the effect achieved by mixing theterphenyl liquid crystal compound becomes greater as the temperature ofultraviolet irradiation increases.

Next, the result of an examination as to the relationship between thecontent of the terphenyl liquid crystal compound and the residualmonomer ratio is described.

FIG. 4 shows liquid crystal compounds which are constituents of thenematic liquid crystal materials used in experiments, the chemicalstructure formula, the composition, and the dielectric anisotropy (Δ∈).Liquid crystal material LC-5 does not contain a terphenyl liquid crystalcompound. Liquid crystal material LC-6 contains 1.0 mass % of terphenylliquid crystal compound A. Liquid crystal material LC-7 contains 12 mass% of terphenyl liquid crystal compound A and 2.5 mass % of terphenylliquid crystal compound B.

In four types of liquid crystal materials, LC-5 to LC-7 shown in FIG. 4and liquid crystal material LC-8 which contains 21.0 mass % of terphenylliquid crystal compound C, 0.3 mass % of a dimethacrylate monomer havinga liquid crystal skeleton was mixed as the photopolymerizable compoundto prepare liquid crystal materials for PSA. A photoinitiator was notused. The resultant liquid crystal materials for PSA were used tofabricate liquid crystal cells which were then irradiated withultraviolet light (365 nm) of about 23 mW/cm² at about 400 mJ/cm² at 20°C., 30° C., 40° C., and 50° C. Thereafter, each of the liquid crystalcells was disassembled, and a small amount of the liquid crystalmaterial was sampled. The sampled liquid crystal material was dilutedwith acetone, and the residual monomer was quantified by a GC/MS method.In the graph shown in FIG. 5, the ordinate axis represents the residualmonomer ratio, which is indicative of the ratio of the amount of theresidual monomer to the initial monomer amount in percentages. Theabscissa axis represents the content of the terphenyl liquid crystalcompound. Note that the present inventor experimentally confirmed thatthe same results are obtained even when a diacrylate monomer was used insubstitution for the dimethacrylate monomer.

As clearly seen from FIG. 5, the residual monomer ratio decreases as thecontent of the terphenyl liquid crystal compound increases. It is alsounderstood that only 1.0 mass % of the terphenyl liquid crystal compoundcontained in the material produces the effect of decreasing the amountof the residual monomer. It is also understood from FIG. 5 that theeffect produced by mixing the terphenyl liquid crystal compound in thematerial increases as the temperature of ultraviolet irradiationincreases. Further, LC-7 of FIG. 5 (terphenyl liquid crystal compound A:12 mass %, terphenyl liquid crystal compound B: 2.5 mass %) and LC-2(terphenyl liquid crystal compound A: 15.0 mass %) and 1C-3 (terphenylliquid crystal compound B: 15.0 mass %) of FIG. 3 result inapproximately equal residual monomer ratios. Thus, it is understood thatthe residual monomer ratio does not depend on the type of the liquidcrystal compound other than the terphenyl liquid crystal compound.

Next, the result of an examination as to the relationship between theamount of the residual monomer and polymeric image sticking isdescribed. The liquid crystal materials used herein are LC-6 whichcontains 1.0 mass % of a terphenyl liquid crystal compound (terphenylliquid crystal compound A), and LC-9 which does not contain a terphenylliquid crystal compound. FIG. 6 shows liquid crystal compounds which areconstituents of LC-9, the chemical structure formula, the composition,and the dielectric anisotropy (Δ∈).

The mean molecular weights of the nematic liquid crystal materialexamples used herein are substantially equal to the molecular weight ofthe monomer, and therefore, the values in mass % can be converted tovalues in mol % without changing the numerical figures. The values aresometimes shown in mol % because the number of monomers remaining in theliquid crystal material is recognized as a significant factor behind theinfluences on the polymeric image sticking and the voltage holdingratio.

First, 0.3 mass % of a dimethacrylate monomer having a liquid crystalskeleton is mixed in LC-6 to prepare a liquid crystal material for PSA.A photoinitiator was not used. The resultant liquid crystal material forPSA was used to fabricate a TFT-based liquid crystal display panel. Thevertical alignment films used were JAIL-204 manufactured by JSRCorporation.

The TFT-based liquid crystal display panel was irradiated withultraviolet light (365 nm, about 23 mW/cm²) at about 35 J/cm² in theabsence of an applied voltage across the liquid crystal layer (theliquid crystal material for PSA). The temperature of the liquid crystaldisplay panel during ultraviolet irradiation was 30° C. The resultantliquid, crystal display panel was disassembled, and the residual monomerwas quantified by a GC/MS method in the same way as described above. Theresults are shown in TABLE 1. The amount of the residual monomer isshown in mass % (=mol %) relative to the nematic liquid crystalmaterial. The presence or absence of application of a voltage foralignment of the liquid crystal molecules does not affect the amount ofthe residual monomer.

TABLE 1 Terphenyl Content Residual Monomer Amount (mass %) (mass % ormol %) LC-6 1.0 0.010 LC-9 0 0.022

As clearly seen from the results shown in TABLE 1, with addition of only1.0 mass % of a terphenyl liquid crystal compound, the amount of theresidual monomer which remains in the TFT-based liquid crystal displaypanel after ultraviolet irradiation at about 35 J/cm² can be reduced to0.010 mass % (=0.010 mol %).

A fabricated liquid crystal cell was subjected to an aging treatment.The aging treatment was carried out by activating the liquid crystalcell placed in a thermostat chamber at 60° C. to display a black andwhite checker pattern over a backlight for a continuous period of 240hours. After this aging treatment, the liquid crystal cell was kept atrest in the thermostat chamber at 60° C. for 240 hours in the absence ofan applied voltage across the liquid crystal layer so that the electriccharge was totally removed away. Thereafter, the presence or absence ofpolymeric image sticking was evaluated by checking the presence orabsence of image sticking of the checker pattern by naked eyeobservation in a dark room environment in a state of display where thedisplay across the entire display surface was at a specific intermediategray scale level (solid display state). The state of display did notchange even when the potential of the counter electrode was changed.With this evidence, it was confirmed as not being DC image sticking.

The results of this evaluation showed that no polymeric image stickingwas found in the liquid crystal display panel fabricated using LC-6,whereas a polymeric image sticking was found in the liquid crystaldisplay panel fabricated using LC-9.

Next, to determine the amount of the residual monomer which constitutesa threshold of polymeric image sticking based on the results shown inTABLE 1, LC-7 was used to fabricate samples of a TFT-based liquidcrystal display panel with different dosages of ultraviolet light (365nm, about 23 mW/cm²), such that the samples contain different amounts ofthe residual monomer. The temperature of the liquid crystal displaypanel during ultraviolet irradiation was 30° C. The vertical alignmentfilms used were JALS-204 manufactured by JSR Corporation. The TET-basedliquid crystal display panel used had the pixel numerical aperture of54% and included an interlayer dielectric film between the pixelelectrode and the bus line whose ultraviolet transmittance was about80%. A plurality of samples were prepared for each condition. Some ofthe samples were used for the quantification of the amount of theresidual monomer. The remaining samples were used for the evaluation ofpolymeric image sticking. The results are shown in TABLE 2.

TABLE 2 Sample No. A B C D Residual Monomer Amount 0.048 0.022 0.0140.012 (mass % or mol %) Polymeric Image Sticking X X ◯ ◯

It is understood from the results shown in TABLE 2 that, to preventoccurrence of polymeric image sticking, the amount of the residualmonomer need to be smaller than 0.015 mass % (=mol %). Also, aspreviously described, when using LC-9 which does not contain a terphenylliquid crystal compound, the amount of the residual monomer can only bereduced to 0.022 mass % even with ultraviolet irradiation of 35 J/cm².Therefore, it is understood that a large effect is produced by adding aterphenyl liquid crystal compound to the nematic liquid crystalmaterial.

As described in the examples presented herein, when 0.3 mass % of thephotopolymerizable compound is mixed in the nematic liquid crystalmaterial, the content of the terphenyl liquid crystal compound ispreferably 1 mass % or more. However, when the content of thephotopolymerizable compound is less than 0.3 mass %, the desired effectsare produced even if the content of the terphenyl liquid crystalcompound is less than 1 mass %. The content of the photopolymerizablecompound which is mixed in the nematic liquid crystal material ispreferably not less than 0.10 mass % and not more than 0.50 mass %relative to the nematic liquid crystal material. If less than 0.10 mass%, the effect of sustaining the alignment of the liquid crystalmolecules may not be sufficiently produced. If more than 0.50 mass %,the amount of the residual monomer may not be sufficiently reduced.

The upper limit of the content of the terphenyl liquid crystal compoundmixed in the nematic liquid crystal material is not limited to aspecific value. However, in terms of the voltage holding ratio (VHR), itis preferably not more than 35 mass % and is more preferably 25 mass %or less.

The nematic liquid crystal materials which contain the terphenyl liquidcrystal compound in different proportions, 14.5 mass % (LC-7), 25.0 mass%, 35.0 massa, and 44.0 mass %, were used to fabricate liquid crystalcells (cell gap: 3.25 μm). A photopolymerizable monomer was not added.The voltage holding ratio immediately after the fabrication of theliquid crystal cells and the voltage holding ratio after the irradiationwith ultraviolet light (365 nm) at 24 J/cm² were measured. Themeasurement of the voltage holding ratio was carried out by applying arectangular wave with the amplitude voltage of ±5 V and the frequency of30 Hz to the liquid crystal cells kept at rest in the thermostat chamberat 70° C. The duration of the voltage application was 60 microseconds.The integral value of the holding voltage for the period of 16.7milliseconds from the start of the voltage application, which isrepresented in percentages relative to that achieved under the conditionwhere the holding voltage was 5 V, is referred to as the voltage holdingratio. The initial values of the voltage holding ratio and the values ofthe voltage holding ratio which were obtained after the ultravioletirradiation are shown in FIG. 7.

As understood from FIG. 7, the voltage holding ratio decreases as thecontent of the terphenyl liquid crystal compound increases. To securethe reliability in an actual use environment, the required voltageholding ratio is 99.0% or higher. In view of the results shown in FIG.7, the content of the terphenyl liquid crystal compound is preferablynot more than 35 mass % and is more preferably 25 mass % or less.

As described above, the terphenyl liquid crystal compound contained inthe nematic liquid crystal material produces the effect of specificallyincreasing the efficiency (reaction velocity) of a photopolymerizationreaction (radical reaction). As a result, in an end-product liquidcrystal display device, the content of the photopolymerizable compoundremaining in the liquid crystal layer relative to the nematic liquidcrystal material is less than 0.015 mol %. Further, the UV dose need notto be increased, but rather can be decreased, as compared with theconventional process. Thus, there are advantages in terms of reliabilityand production cost.

INDUSTRIAL APPLICABILITY

The present invention is preferably used in liquid crystal displaydevices for use in mobile phones, television sets, video game systems,various types of monitors, etc.

1. A liquid crystal display device, comprising: a liquid crystal layercontaining a nematic liquid crystal material; a pair of electrodesopposing each other via the liquid crystal layer; a pair of alignmentfilms respectively provided between the pair of electrodes and theliquid crystal layer; and an alignment sustaining layer formed of aphotopolymerized material on each of surfaces of the pair of alignmentfilms which are closer to the liquid crystal layer, the alignmentsustaining layer being configured to regulate a pretilt azimuth of aliquid crystal molecule of the liquid crystal layer during the absenceof an applied voltage across the liquid crystal layer, wherein thepretilt azimuth of the liquid crystal molecule of the liquid crystallayer is regulated by the alignment sustaining layer during the absenceof an applied voltage across the liquid crystal layer, the nematicliquid crystal material contains a liquid crystal compound having aterphenyl ring system as an indispensable component, and the liquidcrystal layer further contains part of a photopolymerizable compoundwhich is a source material of the photopolymerized material, a contentof the photopolymerizable compound relative to the nematic liquidcrystal material being less than 0.015 mol %.
 2. The liquid crystaldisplay device of claim 1, wherein a content of the liquid crystalcompound having the terphenyl ring system in the nematic liquid crystalmaterial is in a range of not less than 1 mol % and not more than 25 mol%.
 3. The liquid crystal display device of claim 1, wherein thephotopolymerizable compound includes a diacrylate monomer which has aliquid crystal skeleton or a dimethacrylate monomer which has a liquidcrystal skeleton.
 4. The liquid crystal display device of claim 1,wherein the pair of alignment sustaining layers include a particle ofthe photopolymerized material which has a particle diameter of 50 nm orless.
 5. The liquid crystal display device of claim 1, wherein the pairof alignment films are vertical alignment films, and the nematic liquidcrystal material has negative dielectric anisotropy.